Television and like systems



Dec. 31, 1940. E. Q CORK ET AL 2,227,066

TELEVISION AND LIKE SYSTEMS Filed Jan. 14, 1936 zfsneets-sheet 1 fm l 4RAD/o Recs/vela 7a. arc/Lavez HOME? Dec. 31,1940. E. c. 'CORK Erm. Y2,227,066

TELEVISION AND LIKE SYSTEMS Filed Jan. 14, 1936 2 Sheets-Sheet 2 (7a076/1491, /Mvofr l Patented Dec. 31, 1940 2,227

UNITED STATES PATENT OFFICE TELEVISIGN AND LIKE SYSTEMS Edward CecilCork, Ealing, London, Michael Bowman-Manifold,` Worplesdon Station, andCharles Leslie Faudell, Acton, London, England, assignors to Electric &Musical Industries Limited, Hayes, England, a company of EnglandApplication January 14, 1936, Serial No. 59,022 In Great Britain January15, 1935 15 Claims. (Cl. 178-75) The present invention relates totelevision and like systems, and is particularly but not exclusivelyconcerned with the problem of synchronising a scanning operation at areceiver of a television system with theA scanning operation vtakingplace at the transmitter.

It is usual to control the scanning operation at the receiver by meansof impulses generated atV the transmitter; in mostsystems in which, forexample, a cathode ray tube is employed to reconstitute the transmittedpicture, these impulses are caused to `control the generation ofsaw-tooth scanning oscillations.

rFwo sets of synchronising impulses areusually transmitted; an impulse,known generally as a strip impulse, is generated'immediately before orafter the scanning of each strip of the picture to be transmitted, and aframe impulse is generated between complete scans of the picture or, inthe case in which the object `to be transmitted is a cinematograph film,between the scanning of successive frames. The strip impulses aregenerally arranged to be of substantially the same amplitude as theframe impulses. "In order that impulses of one set may be separated atthe receiver fromv those of the other, it is usual to arrange that theframe impulses are oi longer duration than the'kstrip impulses; sincethen the frame impulses contain component frequencies lower than anyinthe strip impulses, the strip impulses may be substantially completelyexcluded from the frame s'ynchronising apparatus at the receiver bymeans of a low-pass filter. `The effect of this filter is to attenuatethe strip impulses relatively to the frame impulses, with the resultthat the impulsesare made to differ considerably in amplitude; they maythen be separated from one another by an amplitude separation, or fed toa saw-tooth oscillation generator such for example as a blockingoscillator which requires an impulse of greater than a criticalamplitude to trigger itv off. i

If it is necessary 'to exclude the frame frequency impulses from thestrip synchronising apparatus at the receiver, vthis may be `achieved bymeans of a high-pass lter.

In systems employing a method such as that outlined above,for separatingone set of synchronising impulses from another, it has been that theoperation of frame synchronising has tended to be `upset by one or moreelements of the low pass filter holding a residualcharge after the stripsynchronising impulse last received has ceased. The reason for this isthat between pulses. 'I'he residual charge adds residual charge held bythe filter results blocking oscillator being triggered-ofi than it wouldotherwise be.

particularly in cases in which interlaced sc a different set of stripsof the object to be successive scans of the object, the strips ofcomplete scan being separated by blank To make this point more clear, itwill differ by one half of a strip scanning will be clear that theresidual charge due preceding strip impulse which is added added to thealternate framing impuls framing impulses thus trigger the blockipicture occur in pairs.

It may `also be foundv that interfering operation of synchronising. Apulse due cause that frame' impulse to take effect frame impulse maytake effect at the ning is employed or not.

Similar problems arise in systems othe N,ttt

the time constant of the separating meansv is at least of the same orderas the shortest interval to .the

charge due to the next succeeding impulse and increases its amplitude.In a case in which the impulses are applied to a blocking oscillator,the

in the earlier The effect referred to manifests itself more anning isemployed, and it tends to produce distortion ofv the reconstitutedpicture. In interlaced scanning,

transmitted is scanned in each oi a small plurality of one of the setsbeing intercalated between the strips of another set. In such cases, thedistortion above mentioned may take the following form: the strips inthe reproduced picture may not be evenly interlaced, but may overlap oneanother, successive groups each comprising a number of strips equal tothe number of part-scans making up a spaces.

be assumed that the object tov be transmitted is scanned completely intwo complete scans; the phases of successive framing impulses must thenperiod with respect to the strip impulses, and it is, convenient toarrange that one framing impulse occurs near to the endV of a strip,while the next occurs near to the middleof a strip period. It

to the to the frame .impulses occurring near to theend of a strip isless than the residual charge which is curring near to the middle of astrip. The latter cillator too soon, and the strips in the reproducedpulses,

such as those due to atmospherics, may upset the tov an atmosphericoccurring just before a'irame impulse may produce a residual chargesuiiicient to earlier correct time; distortion of the reproduced picturemay arise from this cause whether interlaced scanr than The presentinvention accordingly providesa method of separating impulses ofrelatively long duration from impulses of a relatively short Vduration,which method comprises the step of converting said impulses into derivedpulses, the amplitude of each of which increases during a period of timesubstantially equal to the duration of the corresponding impulse to amaximum value which is dependent on said duration, andthen falls to aminimum value at a rate which is greater than that of the increase andsuch that said minimum value is substantially fixed. In order'that oneimpulse shall not affect that which follows it, yit may be necessary tomake the rate at which the amplitude of the derived pulses falls morerapid when the impulses succeed one another closely than when they aremore widely spaced. i

f The invention further provides, for use in television systems, amethod of separating frame impulses of relatively long luration -fromstrip impulses of `relatively short duration, which method comprises thestep of converting said impulses into derived pulses, the amplitude ofeach of which increases, duringa period of time substantially equal tothe `duration of the corresponding impulse, to a. value which isdependent on said duration.- and then falls, ata rate which is greaterthan that of the increase, and is such that, at least in the case of apulse derived from a line impulse, a substantially fixed value isreached beforethe arrival of the next impulse.

According t0 a further feature of the invention, apparatus forseparating impulses of relatively long duration from impulses ofrelatively short duration comprises an impedance element, andcontrolling means adapted to be fed with both long and short impulsesand to cause the chargeA held by said impedance element to be changed inone sense during the impulses and in the opposite sense during theintervals between impulses, .characterised in that the time constantofthe circuit determining the rate of change during impulses is muchlonger than the time constant 'of the circuit determining the rate ofchange duringintervals between impulses. The time constant of thefirst-named circuit may be veormore times that of the second-namedcircuit.

The invention also provides a method of operating apparatus according tothe preceding paragraph, according to which the time constant of thesecond-named circuit is made much shorter than the interval between anyshort impulse and a longer impulse following it. According to anothermethod of operating apparatus according 4to the preceding paragraph, thetime constant ofthe rst-named circuit is made longer than theduration'of a long impulse.

In apparatus according to the invention, the controlling means may bearranged to prevent appreciable charging current flowing in saidimpedance element except when either a long or a short impulse isoperative upon said controlling means. In these circumstances, since theimpedance element only takes appreciable charge during the appliedimpulses, it will be clear that the Vlong impulses give rise to agreater maximum `charge than the short impulses. The impedance elementmay be a condenser, in which case two sets of substantially triangularvoltage "pulses of diiierent amplitude are set up across the` condenseror it may be an inductance coil. In the latter case, the chargingcurrent is conveniently caused to flow in a resistance arrangedin'series with the coil; triangular voltage pulses are then set upacross the resistance. The triangular pulses oflarger amplitude may thenbe separated from those of smaller amplitude by an amplitude selectionfor example. Alternatively, the triangular pulses may be fed directly toa utilising device which in the case of a television system .forexample, may comprise a saw tooth oscillation generator, and which isarranged to be responsive only ,toV the pulses of larger amplitude.

The controlling means is preferably a thermionic valve whichV isarranged to allow the impedance element tocharge up during the time for`which kthe impulses last. The mixed impulses are conveniently fed tothe control grid circuit of the controlling valve. When the impedanceelement isa condenser, the valve is arranged to be conductive, and toprevent the condenser from charging up, during the intervals betweenimpulses, and -it is arranged to be insulating during `the impulses.When an inductance coil is employed, however, the valve is arranged tobe insulating atall `times except when the impulses are operative.

Other yfeaturesrof` the invention will appear hereinafter.

Certain embodimentsof the present invention as applied to televisionsystems will now be described by 4way of example withreference to theaccompanyinggdrawings:` it -will be assumed throughout this descriptionthat it is desired to separate framingimpulses from strip impulses, thelatter having 'the shorter duration; it will alsobe `assumed thatseparation is effected at the-receiver, `and :that the received signal,in which the twosets of impulses appear together, is of the form shownin Fig. l.

In the drawings, Fig. 2 shows schematically, by way ofexamplefa'television receiver, employing aicathode ray tube as apicture-reconstituting device, to which the present invention isapplicable, and Figs. 3 to '7 show various circuit arrangements suitablefor use in the vreceiver of Fig` 2.

Referring to Fig. 1, the line l represents a datum line corresponding toblack in the picture signals 2. The strip impulses 3 and theframing'impulses; 'of which only two, numbered 4' and 5 are shown, areIall ofY substantially the same amplitude and are transmitted in theblackerthan-black sense. The framing impulses are of longer durationthan the strip impulses, and are adapted to give interlaced scanning intwo traversals of the object to be transmitted. The framing impulse 4commences at the end of a strip period, whilst the impulsev 5 commencesat the middle of a strip. The signal of Fig. l is given for purposesofexplanation only, and the invention is kof course not restricted to thisparticular form of signal. For example, a plurality of impulses may betransmitted at the end of lr2,227,066 each frame, and the synchronisingimpulses may be in the same sense as the picture signals, but of greateramplitude.` Furthermore, as has been stated, the invention is notlimited to interlaced scanning; where, however, interlaced scanning thecathode-ray picture-reconstituting device d.

'I'he signal of Fig. 1 is also applied to a selector e which serves tofree the synchronising signals (3, 4 and 5) from picture signals (2),and to feed the synchronising signals to a line-frequency scanningoscillation generator f and to a separator g, the latter serving toseparate frame scanning impulses from line scanning impulses and to feedthe frame impulses to a frame-frequency scanning oscillation generatork. The generator f feeds saw-tooth line-scanning o-scillations toscanning coils h and generator 7c feeds similar oscillations at framefrequencyto coils v.

Referring to Fig. 3, which shows one` form which the apparatus e and gmay take, a screened grid valve having twoanodes 6 and 1, has its anode1 connected through a load resistance 8 to a point in a suitable `anodecurrent source, which is not shown for the sake of simplicity. Allpoints in the gure bearing a positive sign are connected to this source,which has its negative terminal connected to the cathode of valve 5. Theanode 6 of the valve 5 also has a load resistance, not shown, and isarranged tor feed the received synchronising impulses to thestripfrequency scanning oscillation generator f, Fig. 2 which, since thereceived picture is to be reconstituted by `means of a cathode ray tube,may conveniently comprise a saw-tooth oscillation generator such as ablocking oscillator.

It will be noted` that all of the synchronising impulses have an initialsteep transient portion. With the exception of that of impulse 5, andevery alternate framing impulse thereafter 'and therebefore, thesetransient portions recur at the strip scanning frequency, and all ofthesynchronising impulses, including the framing impulses, are fed to thestrip synchronising apparatus. The framing impulses, with the exceptionof those occurringduring the strip periods, thus assist in maintainingthe strip Ascanning operation in synchronism.

The received signal (Fig. 1) is fed to the control grid circuit of valve5 in sucha` sense that the synchronising impulses drive the control gridpotential in the positive direction. The control grid is biasedto apoint just/beyond anode current cut-off, and thus of the receivedsignal, only the synchronising impulses cause anode current to flow, thepicture signals being ineffective.

'Ihe synchronising impulses, free from picture signals, `are se't upacross resistance 8V and are fed through coupling condenser 9 to thecontrol grid circuit of a screened grid valve I0. The control grid ofthis valve is biased to a small positive potential. The amplitude of theSynchronising impulses at the control grid of the valve I is made suchthat each impulse swings the potential of that grid to a value morenegative than that corresponding to anode current cut-off, so

vthat the valve passes no anode current during :the synchronisingimpulses.

'Ihe anode circuit of valve I9 comprises a high resistance II, and twocondensers I2 and I3 are connected in series between the anode and thecathode of this valve.' It will be clear that during the intervalsbetween synchronising impulses, that is, when valve Iii is conducting,most of the current in resistance II flows through valve I0; littlecharge flows to condensers I2 and I3, and a certain small datumpotential difference (which may be arranged to be approximately zero) isset up across condenser I3. However, during an impulse, when valve Illis insulating, the condensers I2 and I3 chargeup, condenser I3 reachingto a potential which exceeds the datum potential, and is dependent,inter alia, upon the duration of the impulse. Furthermore, when theimpulse ceases, the potential diierence across condenser I3 isrelatively rapidly reduced to the datum value, and no residual chargedue to the impulse remains to add to the next succeeding impulse.

In one arrangement, employing a received signal of the form shown inFig. 1, the duration of each of the line impulses is 10 microseconds`(ms), the frameimpulses each have a duration of 4'0 ms., and theminimum interval between successive line and frame impulses is 40 ms.;`in this arrangement, the time constant vgoverning the charging ofcondensers I2 and I3 is approximately 200 ms., while the eiective timeconstant of discharge is approximately 5 ms.,

A series of triangular voltage pulses, the amplitude of each of whichrises during the impulse period to a maximum 'value dependent on theduration of the impulse and then falls at a relatively rapid rate toasubstantially xed minimum value, is thus set up across condenser I 3.The

triangularpulses due to the framing impulses are of greater amplitudethan those due tothe strip impulses, and the `whole series of pulses isfed to a valve I4, which'is arranged to effect an amplitude selection.`t l 'I'he control grid of Valve I4 is biased relatively to its cathodebymeans of biasing resistance I5 and its associated shunt condenser to apoint beyond anode current cut-off, and it is arranged that only thetriangularpulses of greater amplitude are able to causean'de `current toow. There is thus set up in the'output circuit of valve I4 a series ofpulses each of which hasl the same phase as the framing impulsewhich`initiated it. The output circuit of valve I4`is coupledl to a blockingoscillator for example, which constitutes the frame-frequency generator7c, and each pulse in the output circuit of valve I4 is arranged toyinitiate a. return stroke of the saw-toothframe `scanning oscillation.

In the arrangement lshown in Fig; 4, the valve I0', which corresponds-tovalve I0 in Fig. 3,\has a resistance I6f in its grid circuit, anda loadresistance II of highvalue initsvanode circuit. The valve l'may beshunted by a similar screened grid valve (not shown) to feed the linesynchronising apparatus, or a double-anode valve may be employed inplace of valve Ill. The received signal is applied to input terminals I8in such a sense that the picture `signals tend to make the control gridmore positive; the control grid is biased so that picture black makesthe grid slightly more positive with respect to the cathode than )thepotential at which grid current commences to ow, and` the picturesignals accordingly lie within the region of grid current, but owing tothe voltage drop across resistance i6,

the actual grid potential remains substantially constant and unaffectedby the ow of grid current. The synchronising impulses are arranged toreduce the grid potential to a point more negative 'than anode currentcut-off.

' Connected between the anode and cathode of valve HJ are two condensersI2 and I3 arranged in series. Asin the arrangement of Fig. 2, a seriesof triangular pulses is set up across condenser I3, the pulsescorresponding to the framing impulses being of greater amplitude thanthose corresponding to the strip impulses.

The whole series of pulses set up across condenser I3 is applied betweenthe control grid and cathode of a grid-,controlled gas discharge `tubeI9, the grid of which is so highly negatively biased by means of biasingresistance ZIJ and its associated shunt condenser that only the pulsesof greater amplitude set up across condenser I3' are able to initiatelthe iow of anode current in the tube I9.

The anode circuit of the tube I3 comprises coil 2|, resistance 22 andcondenser 23 arranged as shown; the circuit I9, 2I, 22 and 23 isselfoscillatory, and operates to set up a potential difference ofsaw-tooth wave form across condenser 23, and it is arranged that thefrequency of this saw-tooth oscillation is equal to the frequency of thevoltage pulses of greater amplitude, and hence to the framing frequency.The saw-tooth oscillation set up across condenser 23 is employed forscanning purposes. The valve I9 and its associated circuit thusconstitutes the frame-oscillation generator k of Fig. 1.

In a modification of the arrangements of Figs. 3 and 4, the condensersI2 and I3, or I2 and I3 respectively, are connected in series betweenthe anode of controlling valve I0 or I0 and the positive terminal of theanode current source. Now in the arrangements of Figs. 3 and 4, when animpulse arrives, the potential at the junction point of the twocondensers becomesmore posi'.- tive, and the potential difference acrosscondensers I3 and I3 increases, In the modified arrangements, althoughthe potential at the junction point becomes more positive when animpulse arrives, the potential dierence across condensers I3 andI3decreases. The modified arrangements are fundamentally equivalent tothose shown in Figs. 3 and 4, and in both cases the change in the chargeon condensers I3 and I3 maybe regarded as due to the ow of chargingcurrent tending to make the junction point of condensers I2 and I3, orI2' and I3 more positive.

Fig. 5 shows an arrangement in which the synchronising impulses areconverted into triangular voltage pulses of different amplitudes bycausing them to initiate the flow of vcharging current in aninductance.v

Referring to Fig. 5, a screened grid valve 24 has in its anode circuitan inductance coil 25 in series with a resistance 26. Means (not shown)are provided whereby the control grid of valve 24 is so biased,relatively to the cathode thereof, that no anode current flows in theabsence of applied signals, and av received signal (such as that ofFig. 1) is applied to input terminals 2'I in such a sense that only thesynchronising impulses cause anode current to flow, the picture signalshaving no effect. The time constant of the coil 25 and resistance 26 ismade longer than the duration of a framing impulse, and during eachsynchronising impulse, the valve 24 conducts and a current graduallybuilds up in coil 25. At

the end of the pulse, the; valve insulates and the charging current isshut off; triangularvoltage pulses are thus set up across resistance 23.

Means (not shown) are preferably provided,

in the arrangement of Fig. 5, for damping the back electromotive forceset up across the coil 25.

A backed-off diode valve may be employed, if desired, to separate thetriangular voltage pulses of greater amplitude from those of smalleramplitude. An arrangement employing a diode valve f is shown in Fig. 6.1

Referring to Fig. 6, a screened grid valve `2B has an anode resistance29, and a condenser 33 connected between its anode and cathode.

Means (not shown)` are provided whereby thel is applied to the inputterminals 3| in such a sense that the picture signals lie within theregion of grid current, the synchronising impulses reducing the anodecurrent in the valve to zero.

As in the arrangement of Fig. 3, triangular volt- I age pulses are setup across condenser 33.

These voltage pulses are established across a circuit comprising awinding 32 of a three coil transformer, a diode valve 33, and a sourceof rent commences to ow, and the received signal Z0 potentialdifference-33 which is arranged with 30 its positive polev connected tothe cathode of diode 33. vThe potential of the source 35 is greater thanthe potential 4set up at the anode of the diode by the voltage pulses ofsmaller amplitude set up across condenser 33, but is less f than thepotential due to the pulses of greater amplitude. Each ofy these latterpulses thus causes current to flow in the diode 33 and winding 32; in amodified arrangement, which is sometimes found preferable, the positionsof g diode'33 and coil 22 are reversed, the` anode of the diode beingconnected to the anode of valve 28.

The windings 35 and 36 of the transformer form a part of the framefrequency blocking oscillator, which is of a known kind, and comprises ablocking oscillator valve 3l; for the sake of simplicity, it is notproposed to describe either the construction or the method of working ofthis blocking oscillator, since these details are not of primaryimportance from the point of view of the present invention. It willsuffice tol say that the blocking-oscillator can be arranged to generatea saw-tooth oscillation of the same frequency as controlling impulsesapplied to the blocking oscillator valve 31; vwhen current flows indiode 33 and winding 32, due to a pulse of greater amplitude acrosscondenser 33, the impulse which is accordingly set up across winding3I`is fed to the grid circuit of blocking oscillator valve 31, andinitiates a returnstroke of the sawtooth oscillation.

A dry contact rectifier may be employed in place of the valve 33 in thearrangement shown in Fig. 6; preferably the winding 32 of the threecoiltransformer` is electrostatically screened from the remaining windings.

Referring now to Fig. '7, which yshows diagrammatically a preferred formwhich the apparatus e, gand lc of Fig. 2 may take, a screen grid valveil has its anode connected to the positive terminal of a source (notshown) of anode current, the negative terminal 0f which is earthed,through a resistance 42, and, has its control and screen grid circuitscoupled together by means of an iron-cored transformer 43. 'The valve 4|is adapted to operate as a blocking oscillator, and to generatesaw-tooth scanning'oscillations which are taken oi from'a suitablecondenser (not shown) connected between terminals 44. A condenser 45 isconnected in the controlgrid circuit of valve 4|, and the grid of this`valve is connected to its cathode `through a leak resistance 46.

Received television signals of a form such as that illustrated in Fig. 1already referred to, are fed to input terminals 21 and, for example, byan arrangement similar to that disclosed by Fig. 5, are establishedbetween the control grid and cathode of a pentode valve 48 in such asense that the picture signals cause the controlgrid potential toincrease in the positive direction relative to the cathode potential. l

Now the D. C. component of the picture signals is arranged to be presentin the received signals, and in the case of a transmission by modulatedcarrier, it is arranged that the output from the signal detector at thecarrier amplitude corresponding to black in the picture is somewhatpositive relative to earth; valve 48 is connected through a conducivecoupling to the output electrode of the signal detector, and the controlgrid is accordingly biased negatively to a corresponding suitable extentrelatively to its cathode by means of biasing resistance 49, which is inseries with resistances 5l, 52 and 53 between the positive and negativeterminals of the anode current source, and grid current thus flows whilevpicture signals of any amplitude from black upwards are present on thecontrol grid. A highresistance 50 is connected in series with thecontrol grid-cathode path of valve 48, and the now of grid current thusproduces substantially no change in the control grid potential of thevalve. The picture signals accordingly produce substantiallyV no changesin anode current, and the synchronising impulses are thus effectivelyfreed from them.

The anode of valve 48 is connected to the positive terminal of the anodecurrent source through two resistances 54 and 55 in series, and thejunction point of these resistances is -connected through condenser 58tothe inputcircuit of a strip frequency blocking oscillator (not shown)by means of lead 5l. If Aresistances 54 and 55 are made sufficientlylarge,` then, with a suitable screen-grid potential, the synchronisingsignals are freed from picture` signals by anode current limitation, andthe separation is then not dependent upon the flow of grid current. Theresistance 50 is retained in this case, however, and serves to reducethe load on the source of signals due to the input capacity of valve 48.Resistances 54 and 55 may have respectively the values 50,000 ohms and200,000 ohms. y

Current is fed to the screen grid of valve 48 through resistance 58, anda condenser 59 is connected between the screen grid and earth. Inoperation, the synchronising impulses swing the control grid of valve 48negatively to such an exshown) by the valve 48; the latter may arise,for.

example, should the shape of the synchronising signal be such thatcurrent commences to flow in valve 48 before the oscillationg'eneratedby the blocking oscillator ceases. During each synchronising impulseappearing at the control grid of valve 48, condenserv 59 receives acharge. When lthe impulse ceases, the condenser 59 discharges throughthe screen grid-cathode path of Valve 48.

A series `of substantially triangular voltage pulses is thus set upacross condenser 59, the pulses each having an amplitude 'dependent onthe duration of the synchronising impulse to which it is due, and on themagnitude of con-` denser 59. The frame impulses thus produce triangularpulses ofgreater amplitude than do the strip impulses.` r

The whole series of triangular pulses is established between the anodeand cathode of a diode valve 50. The cathode of the diode 60 isconnected to the junction point of resistance 52 and 53 and is thusbiased positively relative toearth, the magnitude of this bias beingmade greater than the steady potential of the anode of diode 60, that`is, the potential of the screening grid of valve 48 in the absence ofvsynchronising signals by an amount such that current owsin diode 89only when a triangular pulse due toa frame impulse is applied to it. Thediode is connected in series with one winding of transformerv 43, andwhen a frame impulse is received on the grid of valve 48, a pulse ofcurrent ows in the diode and in the transformer winding, and theblocking oscillator valve 4l is thus triggered off.

It will be observed that since the valve 48 is a pentode, disturbancesfrom the strip-frequency blocking oscillator which may be fed backalongk lead 5l to the anode of valve llirare prevented by the screeningaction of the'suppressor grid, from affecting the frame frequencyoscillator. Furthermore, disturbances from the frame-frequencyoscillator `are to some extent prevented from` reaching thestrip-frequency oscillator by the diode 60. However, in general, thescanning oscillation generated by the frame-frequency oscillator lastsfor several line-intervals, and during a part of the cycle of thisoscillation, the diode 60 may conduct, and this mayresult in thepotential of the screen grid of valve 48 being reduced to such an extentthat the anode current of this valve is cut oil. Should this happen,strip synchronising impulses are prevented from reaching thestrip-frequency oscillator, whichA may accordingly fallout ofsynclfironism.` This effect may be avoided by giving the screening gridof the valve .48 a suitably chosen high positivepotential, or byconnecting the cathode of diode (illl not to the lower end of thecontrol grid `winding of transformer 43, but to a suitable tapping pointtherein, in which case resistance 52 may be short-A circuited.. l 4

The invention is, of course, not limited to the arrangements described,and many modifications and elaborations'of these arrangements, withinthe scope of the appendedclaims will be apparentV to those versed in theart.

We claim: 1

1.-,In a method of separating impulses of relatively long duration fromimpulses of a relatively short duration, the steps of converting theimpulses into derived impulses, increasing `the amplitudeof each ofthederived impulses in accordance Vwith the duration of the first two namedimpulses, and decreasing theamplitude'of:

relatively long duration from line impulses of relatively shortduration, the steps` of converting the impulses into derived impulses,increasing the amplitude of each of the derived impulses in accordancewith the duration of the rst two named impulses, decreasing theamplitude of the derived impulses at a rate greater than that of therate of increase to a predetermined minimum value, and initiatingsuccessive derived impulses only after the cessation of the precedingderived impulse.

3. Apparatus for separating impulses of relatively long duration` fromimpulses of a relatively short duration, comprising an impedanceelement, means for storing energy in said impedance element, means forcontrolling the time periods during which the storing means is operativeby received impulses, means for enabling the stored energy to change inone sense during the time said impulses are received, and an electricalnetwork having a time constant much shorter than said last nam-ed meansfor enabling the stored energy to change in the opposite sense duringintervals between received impulses.

4. Apparatus as claimed in claim 3 and wherein the time constant of theelectrical network is made shorter than the interval between any shortimpulse and a subsequent long impulse.

5. Apparatus as claimed in claim 3 wherein the time constant of the lastnamed means is made longer than the duration of a long impulse.

6. A television synchronizing system including apparatus for separatingimpulses of relatively long duration from impulses of a relatively shortduration, said apparatus comprising an impedance element, a circuit for.enabling a charge held by said impedance element to change in one senseduring said impulses, a circuit having a time constant much shorter thanthat of said first-named circuit for enabling the charge held by saidimpedance element to change in the opposite sense during intervalsAbetween impulses, controlling means for preventing substantial chargingcurrent flowing in said impedance element except when one of saidimpulses is operative upon said controlling means, means for feedingboth long and short impulses to said controlling means an oscillationgenerator, and means to initiate oscillations of` said generator bypotentials arising from the charge in said impedance element. 7.Apparatus according to claim 6, for use in television systems, whereinsaid controlling means comprises a discharge device, means for feedingcomposite signals comprising picture signals and frame and linesynchronising impulses to said controlling means'and means for operatingsaid discharge device to control the charging of said impedance elementand amplitude separation means to segregate said impulses from picturesignals.

8. Apparatus for separating impulses of relatively long duration, fromimpulses of a relatively short duration, said apparatus comprising acondenser, a thermionic valve having the anode-- cathode path thereofarranged eiectively in shunt with said condenser, means for Vfeedingsaid long and short impulses to said valve, means for causing said pathto be conducting in the absence oi an impulse of either duration andmeans to make said path non-conducting when an impulseis present, and asource of charging current for said condenser.

, 9. Apparatus according to claim 8, comprising a thermionic valvehaving a cathode,v an anode j and a control grid, means including thecondenser for increasing the potential of the control grid in' apositive sense relative to said cathode, and

sense of the potential difference of said source being such that currentflows in said device only when a derived pulse of greater amplitudeexists across said condenser.

l1. A television synchronizing system including apparatus for separatingimpulses of relatively long duration from impulses of a relatively shortduration, saidl apparatus comprising an inductance coil, a thermionicvalve having the anode-cathode path thereof arranged eiectively inseries with said coil, means for feeding said long and short impulses tosaid valve, means for rendering said path non-oonducting in the absenceof an impulse of either duration and to become conducting when animpulse is present, a source of charging current for said coil, anoscillation generator, and means to initiate oscillations of saidgenerator in accordance with a flow of current through said inductancecoil. f

12. A television synchronizing system including apparatus for separatingimpulses of relatively long duration from impulses of a relatively shortduration, said apparatus comprising an inductance coil, a thermionicvalve having the anodecathode path thereof arranged effectively inseries with said coil, means for feeding said long and short impulses tosaid valve, means for rendering said path non-conducting in the absenceof an impulse of either duration and to become conducting when animpulse is present, a source of charging current for said coil, aresistive impedance adapted to be traversed by said charging current, anoscillation generator, and means to initiate oscillations oi saidgenerator in accordance with the iiow of current through said inductancecoil.

13. Apparatus according to claim 12,.compris` ing a thermionic' valvehaving a cathode, an anode and a control grid, means for causing derivedvoltagerpulses set up across said resistive impedance to cause thepotential of said control grid to increase in the positive senserelative to said cathode, and means for biasing said control'gridrelative to said cathode to anegative potential which is of such amagnitude that, in operation, only those voltage pulses which correspondto longer impulses cause anode curr-ent to flow.

14. Apparatus according to claim l2, comprising aunidirectionally-conducting device, a source of' potential difference inseries with said device, and means for establishing derived voltagepulses set up across said resistive impedance across said device andsaid source in series, the magnitude and sense ofI ,the potentialdifference of said source being such that current flows in said deviceonly'when a derived pulse of greater amplitude exists across saidresistive impedance.

15. In a television system in which synchronizing impulses of twodiierent time durations but with equal amplitudes are transmittedsequentially with picture signals, the method of synchronizing a picturereproducer at a. receiving point, which comprises the steps of receivingthe picture signals and both the synchronizing impulses, separating thepicture signals from the synchronizing impulses, converting thesynchronizing impulses into impulses having diierent amplitudesdetermined by the time duration of the corresponding impulse, separatingthe lower amplitude converted synchronizing impulses from 10 the higheramplitude converted synchronizing impulses, initiating energy iiow of a.first source of wave energy by the separated lower amplitude impulses,initiating energy flow of a second source' of Wave energy by theseparated higher ampltude impulses, and controlling the picturereproducer by both of the controlled sources of wave energy.

EDWARD CECIL CORK.

MICHAEL BOWMAN-MANIEOLD.

CHARLES LESLIE FAUDELL.

